Yes graphite is better than copper, at least on steel. You are right about flush pressure being a factor. Generally a pressure of less than 6 or 8 psi works best, with smaller electrodes deflection can be a problem with even these pressures. It is usually best to start low, 1 to 2 psi when the electrode is in the cut, pressure should drop to almost 0 when your electrode retracts. Try increasing in small increments till you achieve the best burn. Some times a pressure of 1/8 to 1/4 psi is best, you need an accurate pressure guage and a good manifold set up with a by pass to achieve good control at these low pressures.
A brief description of the theory on low wear burning. A setting with more than 50 % on time, and as high as 90% on time, combined with low flush pressures contributes to a plating of the electrode by the steel being burnt. Your are burning on the verge of a dc arc. A fine tuning of all burn parameters can lead to wear rates as low as 2 to 5%. It has its draw backs, very fine finishes are tough to accomplish. But over all accuracy is impressive even with only 3 hits. Shallow burns can even be done in only 2 hits.
As a general guide square inches equal on time, .25 sq in 25us on time, these are only ball park , your surface finish requirements may dictate a lower value, If I understand your electrode size of .75 x 2.0 square inches = 1.5 so a value of 150 us on time would be the ideal, but surface finish would be about 150 rms, kinda rough so a setting of maybe 45 or 50 would be more appropriat with an equal ammount of off time, 50% duty cycle to start. Over burn would be about .002 and drawing about 4 amps. You will need to determine how many transistors to turn on to achieve this with your burner. I have some charts on overburn, surface finish, power required, ect, though not machine specific they seem to provide a good start on other machines, time and patience are required to determine what your machine likes, time well spent when you can do more in fewer hits, especially if the carbons are intericate.
Tom Rogers  |